Method and device for detecting a specific spectral feature

ABSTRACT

A method and device for detecting a narrow spectral feature in a sample is described. The method includes the steps of providing a beam of light having an optical frequency bandwidth which is narrow compared to the width of the narrow spectral feature and having a center frequency ω C  which lies near the narrow spectral feature, phase modulating the beam of light with a single RF frequency to provide a pure FM spectrum having upper and lower sidebands, exposing the sample containing the narrow spectral feature to the modulated light so that only one of the FM sidebands probes the narrow spectral feature, photodetecting the light emerging from the sample to detect a RF beat at the specific RF frequency used for phase modulation, and electronically monitoring the amplitude of the RF beat signal to indicate the strength of the narrow spectral feature. A preferred embodiment of this invention is a multiplex readout device for hole burning memories. The device includes means for using a frequency modulated laser light with widely spaced sidebands arranged so that only one of each pair of upper and lower sidebands probes the frequency location of each hole. The device uses many simultaneous RF frequencies to drive the phase modulator and produce the light with FM sidebands which simultaneously probes all the hole locations of interest. The device includes photodetection means to receive the light after it has passed through the sample and phase sensitive multiplex analyzing means to process the electrical signals from the photodetection means to indicate the presence or absence of holes.

DESCRIPTION Technical Field

This invention relates to spectroscopy and more particularly to a methodand apparatus for detecting one or more spectral features in a sample.

It is a primary object of this invention to provide an improved methodand apparatus for detecting a single narrow spectral feature.

It is another object of this invention to provide an improved method andapparatus for detecting a plurality of narrow spectral features.

It is still another object of this invention to provide an improvedmethod and apparatus for detecting a plurality of narrow spectralfeatures simultaneously.

It is yet still another object of this invention to provide an improvedmethod and apparatus highly sensitive to the presence of a narrowspectral feature.

It is a further object of this invention to provide an improved methodand apparatus for the rapid multiplex readout of a plurality of narrowspectral features without requiring any tuning of a laser.

Background Art

Several different approaches have been reported in the literature fordetecting a restricted type of spectral feature, that is, a singleabsorption line in a sample. One of these methods utilizes theappearance of RF beat frequencies when the FM sideband structure offrequency modulated laser light is distorted to lock the centerfrequency of a FM laser to gain line center. This is described by S. E.Harris et al in the Applied Physics Letter, Vol. 7, page 185, 1965. Inthe Harris et al approach, all of the FM sidebands are contained withinthe feature of interest, i.e. the gain line profile. Harris et alpropose to utilize the RF beat frequency including both FM sidebandsonly to locate the center of a very strong line. This method is notsuitable for weak absorption lines or for dispersions, that is, opticalphase shifts, that are separately measured. This method does not teachor suggest a multiplex readout feature.

Another approach is described by M. Cardona in Modulation Spectroscopy,Academic Press, New York, 1979. This approach involves sinusoidallyfrequency-swept incoherent light utilized for some period of time toprobe weak absorptions in solid state samples. In this approach, an RFsignal is obtained which is proportional to the derivative of theabsorption profile. This technique uses incoherent light and, hence,there is no heterodyne amplification of the signal. This method alsorequired a monochromator to provide spectral resolutions and, as aresult, spectral resolution finer than 30 GHz is not achieved. Thisapproach is not suitable for multiplex readout since incoherent light isused.

Another approach is described by C. L. Tang et al in the Journal ofApplied Physics, Vol. 45, page 4503, 1974.

The Tange et al approach involves a sinusoidally frequency-swept dyelaser which is utilized to probe weak absorption in solid state samples.In this case, since a laser is used, spectral resolution of 10 MHz canbe obtained. This technique uses a dye laser spectrum which is not apure FM spectrum and, as a result, there is no heterodyne amplificationof the signal. This technique cannot directly measure the absorption ordispersion since it only measures the derivative of the absorption. Withthis approach a multiplex readout is impossible.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of this disclosure:

FIG. 1 is a schematic diagram of the device;

FIG. 2 is a diagram showing the relative positions of hole locations andFM sidebands in one embodiment.

FIG. 3 is a diagram showing the relative positions of hole locations andFM sidebands in a second embodiment;

FIG. 4 is a diagram showing the relative positions of the absorptionline and FM sidebands.

DISCLOSURE OF THE INVENTION

For further understanding of the invention and of the objects andadvantages thereof, reference will be had to the following descriptionand accompanying drawings, and to the appended claims in which thevarious novel features of the invention are more particularly set forth.

A method and device for detecting a narrow spectral feature in a sampleis described. In this invention, a narrow spectral feature is defined asany optical property of the sample, such as absorption, transmission,reflectivity, index of refraction and optical length which variesrapidly with the optical frequency. Specific narrow spectral featureswhich will be described in detail hereafter are (a) a narrow absorptionline and (b) holes burned in a broad spectral feature such as aninhomogeneous absorption band. The method includes the steps ofproviding a beam of light having an optical frequency bandwidth which isnarrow compared to the width of the absorption line and having a centerfrequency ω_(C) which lies near to the line, phase modulating the beamof light with a single RF frequency to provide a pure FM spectrum havingupper and lower sidebands, exposing the sample containing the absorptionline to the modulated light so that only one of the FM sidebands probesthe absorption line, photodetecting the light emerging from the sampleto detect a RF beat at the specific RF frequency used for phasemodulation, and electronically monitoring the amplitude of the RF beatsignal to indicate the strength of the absorption line. A preferredembodiment of this invention is a multiplex readout device for holeburning memories. The device includes means for using a frequencymodulated laser light with widely spaced sidebands arranged so that onlyone of each pair of upper and lower sidebands probes the frequencylocation of each hole. The device uses many simultaneous RF frequenciesto drive the phase modulator and produce the light with FM sidebandswhich simultaneously probe all the hole locations of interest. Thedevice includes photodetection means to receive the light after it haspassed through the sample and phase sensitive multiplex analyzing meansto process the electrical signals from the photodetection means toindicate the presence or absence of holes.

Best Mode for Carrying Out The Invention

An embodiment will now be described in terms of the device and methodfor the multiplex readout of information encoded by the presence orabsence of photochemical holes burned in an inhomogeneous absorptionband. As shown in FIG. 1, the first step is to provide a narrow bandlaser 10 having an optical frequency ω_(C). Examples of a narrow bandlaser are a single frequency dye laser and a fixed single mode frequencysolid state laser. The carrier frequency 29, ω_(C), is chosen to lie atthe center of the inhomogeneous absorption band 11 as shown in FIG. 2.The inhomogeneous absorption band with holes is one example of aspectral feature whose absorption and index of refraction vary withoptical frequency. The laser has a bandwidth which is narrow compared tothe desired resolution of the information in the spectral feature, thatis, the width of the photochemical holes. The laser passes light intothe phase modulator 12 which modulates the light from the laser sourceto provide a plurality of widely spaced pairs of FM sidebands that arearranged so that only one of each pair probes the optical frequency ofeach hole. An example of a modulator 12 is an electro-optic phasemodulator which is commercially available. Electronic means 14 drivesthe phase modulator 12 simultaneously with a plurality of different RFfrequencies to produce light with FM sidebands which simultaneouslyprobe all hole locations of interest in the sample 16 as shown in FIG.2. An example of electronic means 14 is a frequency comb generator. Themodulated laser beam passes through the sample 16 containing theabsorption band with holes therein and impinges on a fast photodetector18.

A bit of information in the sample 16 is encoded by the presence orabsence of a hole at a location which corresponds to a particular FMupper sideband. For example, the FM upper sidebands 22, 24 and 26correspond to the holes 28, 30 and 32 respectively. The presence of ahole will cause a differential absorption or phase shift to beexperienced between the upper FM sidebands 22, 24 and 26, and the lowerFM sidebands 21, 23 and 25 respectively which correspond to the holelocation. Such a differential will produce a heterodyne amplified beatsignal at the corresonding RF frequency at the photodetector 18, whileif there is no differential, no beat signal will be produced.

The photodetector 18 electrical signal, which simultaneously containsthe different RF frequencies which correspond to the holes which arepresent, is processed by phase sensitive multiplex analyzing electronics20. An example of a photodetector 18 is a solid state PIN diode. Thedesign of electronics 20 is straightforward and well within the state ofthe art. An example of the electronics 20 is a parallel array of doublebalanced mixers, each of which is driven by a local oscillator at one ofthe RF frequencies used to modulate the laser. The analyzing electronics20 discriminates among the different hole locations on the basis of thedifferent RF frequencies, and isolates the signals due to differentialabsorption from those due to differential phase shift by comparing thephase of each RF beat signal to the phase of the corresponding RFdriving frequency of the modulator. If there are a large number of holelocations, it is advantageous to use only that portion of the beatsignal which is due to the differential absorption, since the combinedphase differentials caused by the presence of the holes at otherfrequency locations can cause a spurious differential signal. The lengthof time necessary for the multiplex readout is of the order of Δω⁻¹,where Δω is the typical frequency spacing between hole locations.

By using this method and/or device, all of the hole locations can besimultaneously probed and multiplex readout achieved without tuning thelaser frequency or any of the RF frequencies. With this method, theentire multiplex readout of 100 holes can be accomplished in 10 nsec,corresponding to a data rate of 10¹⁰ bits per second.

Another advantage with this method is the heterodyne amplificationeffect which produces the RF beat signals with amplitudes proportionalto the geometric mean of the intensity of the carrier at ω_(C) and theFM sideband. This allows the hole locations to be probed with relativelyweak optical power densities, thus eliminating undesirable further holeburning during readout.

This method also provides an additional advantage in that the beatsignals appear at RF frequencies where the noise power of any welldesigned single axial mode laser will be very low and approach thequantum limit. This means that if N photons in a particular sideband areused to probe a particular location on the absorption profile, adifferential absorption as small as one part in the square root of N canbe detected. Thus, if a conservative number such as 10¹⁴ photons areassumed to be in the sideband, a differential absorption as small as onepart in 10⁷ can be detected. This represents roughly a factor of 10⁴advantage over conventional direct absorption spectroscopy done withslowly tuned lasers, which is limited by l/f noise to sensitivitieswhich are 10⁵ worse than the quantum limit.

An alternative embodiment as shown in FIG. 3 utilizes the same elementsas described above. The only differences are that the carrier frequency34, ω_(C), is chosen to lie near to, but not on, the inhomogeneousabsorption band 35 and that the RF frequencies are chosen so that FMupper sidebands 36, 38, 40, 42, 44, 46, 48, 50 and 52 overlap the entireinhomogeneous absorption band 35. FIG. 3 shows the relative positions ofthe holes 54, 56, 58, 60, 62 and 64 and the FM sidebands. A bit ofinformation is encoded by the presence or absence of a hole at alocation which corresponds to a particular FM upper sideband. If no holeis present the maximum differential absorption or phase shift will beexperienced between the upper FM sidebands 36, 38, 40, 42, 44, 46, 48,50 and 52 and the lower FM sidebands 66, 68, 70, 72, 74, 76, 78, 80 and82 which correspond to the hole location and thus the maximum beatsignal at the corresponding RF frequency will be produced. The presenceof a hole will produce a smaller differential and hence a smaller RFsignal.

Industrial Applicability

This invention is much broader than the embodiment dealing withmultiplex readout for optical memories based upon photochemical holeburning. This invention provides a basis for spectroscopic diagnosticinstruments, for example, for gas phase chemical reaction or molecularbeam epitaxy. For example, this invention describes a method ofdetecting a specific narrow absorption line which is characteristic of aparticular component in a sample. The first step in this method isproviding a narrow beam of light having an optical frequency bandwidthwhich is narrower than the width of the absorption line 84 and which hasa center frequency 86 ω_(C), which lies near the line 84 as shown inFIG. 4. The next step is to phase modulate the beam of light with asingle RF frequency to provide a pure FM spectrum having upper 88 andlower 90 sidebands. An example of the electronic means used to drive thephase modulator is a sweep oscillator. In an alternative embodiment thesweep oscillator enables the RF frequency to be swept, that is, variedcontinuously, to provide a spectrum of the spectral feature. The sampleis exposed to the modulated light so that only one of the FM sidebands,88 in FIG. 4, probes the absorption line. The light emerging from thesample is then photodetected to detect an RF beat at the specific RFfrequency used for phase modulation. The amplitude of the RF beat signalis electrically monitored as its frequency is varied to indicate thestrength of the absorption line.

While I have illustrated and described the preferred embodiments of myinvention, it is understood that I do not limit myself to precise stepsherein and the right is secured to allow changes and modificationscoming within the scope of the invention as defined in the appendedclaims.

I claim:
 1. A method of detecting a single narrow spectral feature in asample comprisingproviding a beam of light having an optical frequencybandwidth which is narrow compared to the width of the narrow spectralfeature and having a center frequency ω_(C) which lies near the feature;phase modulating the beam of light with a single RF frequency to providea pure FM spectrum having upper and lower sidebands; exposing the samplecontaining the narrow spectral feature to the modulated light so thatonly one of the FM sidebands probes the narrow spectral feature;photodetecting the light emerging from the sample to detect a beat atthe specific RF frequency used for phase modulation; and electronicallymonitoring the amplitude of the RF beat signal to indicate the strengthof the narrow spectral feature.
 2. A method as described in claim 1including monitoring the phase of the RF beat signal.
 3. A method asdescribed in claim 1 whereby the phase modulating RF frequency is sweptto provide a spectrum of the narrow spectral feature.
 4. A method ofdetecting and recovering additional information contained in a broadspectral feature in a sample whose absorption and index of refractionvary with time and optical frequency comprisingproviding a beam of lighthaving an optical frequency bandwidth which is narrow compared to thedesired resolution of the information in the broad spectral feature;phase modulating the beam with a plurality of RF frequencies to providea plurality of FM spectra having upper and lower sidebands which arewidely spaced compared to the bandwidth of the original beam of light;exposing the sample containing the broad spectral feature to themodulated light so that one of each pair of FM sidebands probes thebroad spectral feature at a different optical frequency location,photodetecting the light emerging from the sample to simultaneouslydetect beats at the specific RF frequencies used for phase modulation;and electronically monitoring the amplitude of each RF beat signal as afunction of time.
 5. A method as described in claim 4 includingmonitoring the phase of each RF beat signal.
 6. A method as described inclaim 4 whereby the RF frequencies are chosen so that each narrowspectral feature contained in the broad spectral feature is probed byone of each pair of FM sidebands.
 7. A method as described in claim 4whereby the optical bandwidth of the beam of light is centered at afrequency position within the broad spectral feature.
 8. A method asdescribed in claim 4 whereby the optical bandwidth of the beam of lightis centered at a frequency position outside the broad spectral feature.9. A device for detecting a narrow spectral feature in a samplecomprising:a laser source having a bandwidth narrower than the width ofthe narrow spectral feature and having a center frequency ω_(C) whichlies near a selected narrow spectral feature, phase modulator means formodulating the light from said laser source to a pure FM spectrum havingupper and lower sidebands, means for driving said modulator means with asingle RF frequency to produce an FM sideband which probes the selectednarrow spectral feature in the sample, photodetection means to receivethe light after it has passed through the sample, and electronicdetection means which is capable of monitoring the intensity of the RFelectrical signals from said photodetection means to indicate thestrength of the selected narrow spectral feature.
 10. A device asdescribed in claim 9 wherein the laser source means is a singlefrequency laser.